1. Field of the Invention
The present invention generally relates to image forming apparatuses, and more specifically, to an image forming apparatus having a recording head configured to jet a liquid drop.
2. Description of the Related Art
As an image forming apparatus such as a printer, facsimile, copier, plotter, or a multiple function processing machine including the printer, facsimile, copier, and the plotter, an inkjet recording apparatus is known. The inkjet recording apparatus is a liquid jet recording type image forming apparatus using a recording head configured to jet an ink liquid drop.
In this liquid jet recording type image forming apparatus, the ink liquid drop is jetted from the recording head onto a conveyed sheet so that image forming such as recording or printing is performed. In the liquid jet recording type image forming apparatus, there are two kinds of image forming apparatuses. One is a serial type image forming apparatus configured to jet a liquid drop so that an image is formed while a recording head moves in a main scanning direction. The other is a line type image forming apparatus using a line type head whereby a liquid drop is jetted while the recording head does not move so that an image is formed.
Hereinafter, the “image forming apparatus” means an apparatus configured to jet liquid onto a medium such as a paper, thread, fiber, leather, hides, metal, plastic, glass, wood, or ceramic so that images are formed. The image forming apparatus includes a mere liquid jetting apparatus. In addition, “image forming” means not only providing an image of characters, figures, or the like on the medium but also providing an image such as a pattern having no meaning on the medium. “Image forming” includes adherence of the liquid drop onto the medium.
Furthermore, “ink” is not limited to the recording liquid or the ink and any liquid that is a fluid when being jetted can be applied to the liquid such as fixing liquid. In addition, “sheet” is not limited to a paper but includes an OHP sheet or leather. In other words, the sheet means a subject where the ink drop is adhered. The sheet includes a recorded medium, a recording medium, a recording paper, and a recording sheet.
As a liquid jetting head (liquid drop jetting head) used as a recording head, a piezo-electric type head or a thermal type head are known. In the piezo-electric type head, a vibration plate is displaced by a piezo-electric actuator and the volume in a liquid room is changed so that pressure is increased whereby a liquid drop is jetted. In the thermal type head, a heating element for heating based on electrification is provided in the liquid room so that the pressure in the liquid room is increased by air bubbles generated by heating with the heater whereby the liquid is jetted.
In the above-mentioned liquid jet type image forming apparatus, increasing the number of nozzles or heads has been attempted for accomplishing high speed printing.
Recently, a line type image forming apparatus has been suggested where plural short heads are joined so that a long head array unit is formed whereby an image can be formed without making the head scan. In addition, as a way for responding to the high speed requirement, it has been suggested that the ink jet frequency be increased.
However, increasing the number of nozzles or high speed driving encourages temperature increase of the head. That is, when the temperature of the head is increased, the temperature of the ink inside the head is increased. As a result of this, viscosity of the ink is changed so that jetting properties of the head are influenced. Because of this, in the conventional image forming apparatus, an ink jetting signal or the like is controlled based on the temperature of the head in order to maintain the jetting state constant.
However, in a case where a head array unit having a large number of the nozzles is driven at high speed, since the temperature increase is drastic, it is not possible to adequately respond by only controlling the ink jetting signal.
For example, Japanese Laid-Open Patent Application No. 2006-181949 suggests the following techniques. That is, inside a fixing member configured to support a head board of a line head as a head array unit, an independent liquid path is provided from a common liquid room where jetting liquid is supplied. With this structure, the liquid is circulated so that the temperature of the head is maintained constant.
However, in the apparatus suggested in Japanese Laid-Open Patent Application No. 2006-181949, ink and a liquid substance different from the ink are respectively supplied to the head part via a large number of tubes. Accordingly, the tube structure is complex. In addition, only the temperature of the head can be controlled by the liquid substance.